The present invention relates to semiconductor devices, and more particularly to treating a die receiving area of a lead frame to prevent solder over flow during solder die bonding.
Lead frames often have a die pad to which an integrated circuit die is attached. For high power devices, solder is usually used to attach the die to the die pad because of its good thermal/electrical conductivity and ability to absorb stresses due to coefficient of thermal expansion (CTE) mismatches. The solder joint between the lead frame and the silicon die provides a high thermal dissipation of the heat released by the silicon die and a strong resistance of the thermal fatigue.
Lead frames are formed of a conductive metal such as copper or a copper alloy. It is also common to plate the lead frame such as with Silver or Palladium to improve die attach quality, and it is further known to spot plate areas of the lead frame like the lead frame fingers and the die pad, in order to improve the die attach quality.
Solder over flow is a common problem encountered in solder die bonding. One method of preventing solder over flow is with a moat formed between the die receiving area and the surrounding area. FIG. 1 is a top plan view of a die receiving area 10 of a lead frame having a die 12 attached to a surface thereof. A moat 14 surrounds the die 12. Unfortunately, the moat 14 does not always prevent solder from flowing from below the die 12 and into the surrounding die receiving area. Solder over flow 16 is shown in FIG. 1. The moat design is limited by the ratio of die size to die pad size. Such over-flow solder 16 induces stress on the package and weakens the adhesion between the lead frame and a plastic compound that is molded over the die later in the packaging process.
The wetting ability of molten solder with the lead frame surface influences the flow of solder during die bonding. Conventional lead frames used for solder die bonding have uniform surfaces. That is, all parts of the die pad have the same wetting ability with solder, thus the molten solder flows freely around the die pad when bond pressure is applied.
Accordingly, an object of the present invention is to provide a lead frame with improved solder over flow control and a method for forming the lead frame.